Regenerative glass furnaces operate at very high process temperatures due to a high temperature air preheat level. For achieving higher thermal efficiency, the combustion air is preheated to approximately 2400° F. in ceramic regenerators before reacting with fuel inside the furnace interior. Many methods have been suggested for reducing nitrogen oxides (NOx) emissions from regenerative glass furnaces, but few have been actually implemented.
During high temperature combustion as observed in regenerative glass furnaces, NOx is formed primarily by thermal oxidation of nitrogen in combustion air—denoted as thermal NOx. Generally, thermal NOx depends on time-temperature history of the flame and increases with increasing peak flame temperatures.
The primary methods of reducing peak flame temperatures are reducing the air preheat level, and reducing the firing rate in certain locations of the furnace and using electric boost. Both approaches have limitations due to the lowering of the furnace efficiency, and production rate, and are often difficult to implement due to furnace design considerations.
Pollution control techniques usually result in added cost. Therefore, glassmakers must select the most cost-effective technique available that meets regulated emission limits in order to remain competitive. In some cases, the NOx legislation applies only to new or rebuilt furnaces, whereas, in other instances, it applies to currently operating furnaces. The most common flat glass furnaces are regenerative side-port furnaces that typically produce around 600 ton/s day glass and consume 150 MM Btu/Hr of fuel. These furnaces produce 12 to 20 lb NOx per ton of glass without any NOx abatement technology. Under the Clean Air act of 1990 in the United States, most regions of the United States limit NOx emissions to 2 to 8 lb/ton from large regenerative furnaces. Thus, most glassmakers are required to choose emission control technology for meeting their local emission targets.
There are several known technologies for NOx reduction in regenerative glass furnaces. Including fuel-rich firing, gas reburn on the exhaust side of the furnace, and oxy-fuel firing where nitrogen as the primary source of NOx is eliminated. See the following references, which are incorporated herein by reference:    1. “Controlling Glass Furnace NOx with Gas Reburn”, Ceramic Bulletin, February 1998, pp 51-56 (R. Koppang, A. Marquez, D. Moyeda, M. L. Joshi, P. Mohr and R. Madrazo).    2. “References: “Cost—Effective NOx Reduction Using Oxygen—Enriched Air Staging (OEAS) on Regenerative Glass Furnaces,” Presented at 55th Conference on Glass Problems, Columbus, Ohio, Nov. 8-9, 1994 (M. L. Joshi, D. B. Wisnick, S. K. Panahi, H. A. Abbasi, R. E. Grosman, R. F. Madrazo, W. H. Benz, A. G. Slavejkov, and L. W. Donaldson).    3. U.S. Pat. No. 5,203,859, Oxygen-enriched combustion method, Khinkis et al., Apr. 20, 1993    4. Pilkington Technology Datasheet 2, “Float Glass Technology”. (http://www.pilkington.com/resources/datasheet2float.pdf)
Glass makers are looking for a cost-effective NOx reduction technology which is retrofittable to regenerative furnaces, and may provide one or more of the following benefits: lower peak flame temperatures; reasonable flame lengths due to strategic firing side oxidant injection; complete burnout of CO and combustibles; NOx emission reduction; and improved furnace efficiency due to significant combustion taking place within the melter.